When Portland Cement hydrates calcium silicates and the calcium aluminates crystalize with the water and form the crystals which provide the desired strength of the end product. The reaction liberates a certain amount of free line as calcium hydroxide, and this amount of free lime generally falls somewhere between 12 and 18 percent of the Portland Cement. The reason for the variation is that there is variation in the raw materials used at any particular mill which results in the manufacture of higher or lower calcium content.
This free lime is usually deleterious to a finished mortar or concrete volume for a number of reasons which have been well described in the literature as resulting in unsoundness, and/or because the lime leaches to the surface and causes efflorescence. Not all of the deleterious effects of free lime are considered deleterious in all forms of concrete. For example, resistance to sulphate attack is very important for a sewer pipe but usually not for a cement block, while efflorescence is not a problem in sewer pipes but can be a real problem in cement block. Nevertheless, the removal of some or most of the free lime, as free lime, is almost never undesirable and almost always desirable if not vital to the quality of the final product. The literature describes many additives which will combine with the free lime to form insoluble salts. Two of these also add cementitious values which means that the Portland Cement which must be introduced into the mix can be reduced somewhat for equal strength. One of these is the addition of some finely ground siliceous material such as silica flour, but if it has not been calcined, no reaction will occur between it and the free lime unless the product is subsequently autoclaved at high temperature and high steam pressure, i.e. treated under hydrothermal conditions. However, if the siliceous material is pozzolanic i.e., of volcanic origin, or fly ash, or the ash from which shale oil has been extracted, or some other siliceous or argillaceous material, then reaction will occur between the lime and the pozzolan over a period of time at ambient temperature, which reaction will cause the formation of calcium silicates which will not only eliminate much of the deleterious free lime but in addition will add cementitious strength.
This enables a pozzolan to replace a part of the more expensive Portland Cement at no sacrifice in eventual strength of seasoned concrete, and due to the lowered free lime content in some circumstances the concrete quality will actually be improved with regard to some characteristics. Pozzolans have been used as an ingredient in concrete and masonry products since the Romans, and since they can be taken from different sources they can differ in their chemical and physical characteristics and therefore vary in their performance as pozzolans.
Fly ash is usually an extremely effective pozzolan but this depends somewhat upon its source. Moreover, fly ash is a most economically desirable pozzolan in the United States because it is not only a waste product but also a pollutant. In addition, fly ash almost always possesses additional advantages, due to its particle size and shape, which are not normally associated with other pozzolan.